0
\$\begingroup\$

I am looking to actuate a 12V SPDT relay depending on weather or not a 12V PWM signal is received or not. Headlamps use a reduced power PWM signal for daytime running lamps. I want to be able to detect that signal separate from a 12VDC source (normal headlamp operation) and actuate a relay based on it to relocate that daytime running lamp to another location. I am an ME by trait but have limited experiences with circuits.

I have been able to use a simple RC circuit to turn that PWM into a lower constant voltage. Trouble is, I don't know how to compare the full 12V and the reduced voltage and also still carry enough amperage to control a relay coil.

Looking for something as simple as possible. Some have suggested using a using a 555 as a retrig monostable missing pulse detector or an RC circuit with a comparator. I don't know if either of these would work or how to specifically design them. Any help would be greatly appreciated!

\$\endgroup\$
0
\$\begingroup\$

I'd recommend breaking your design into two stages:

1 - Detect the state of your PWM signal,

2 - Drive your relay coil accordingly

For the first stage, I'd recommend an op-amp comparator to determine the state of the PWM signal. Converting the PWM signal to a DC level is a good start towards accomplishing this. Basically, you want your comparator to give a 'high' output if the PWM signal is above some threshold, or a 'low' state if it's below the threshold. You can set the threshold voltage using a simple 2-resistor voltage divider from your 12V supply.

For the second stage, I'd recommend an N-type FET to serve as a low-side switch for the relay coil. When the output of the comparator goes 'high', the NFET conducts, and current flows through the relay coil to activate it. When the comparator goes 'low', the NFET stops conducting, and the relay is deactivated.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the detailed answer! I should be able to fumble through the circuit design but if you're so inclined (again, I know I'm asking a lot) any details or circuit diagrams are always appreciated! I'll model everything in LTspice before I buy components but you might save me quite a bit of time and headache having to guess some values. \$\endgroup\$ – C Hamilton Oct 26 '18 at 14:47
  • \$\begingroup\$ Any idea what Op-Amp Comparator might work best in this circuit? Something I can model in Lt-spice? \$\endgroup\$ – C Hamilton Oct 26 '18 at 16:48
  • \$\begingroup\$ I would detect the signal state with a couple of retriggerable monostable multivibrators (NE555 or 74xx126, both of them reviled but OK in this application). Set up to detect the correct polarities, you can have an "on at least sometimes" signal out, and an "off at least sometimes" signal out. Then it should take a few logic gates to determine whether it's the headlights or running lights that are on. \$\endgroup\$ – TimWescott Oct 26 '18 at 18:06
0
\$\begingroup\$

You are worrying about a condition which is not necessarily a problem. Basically, you're concerned that you might not be able to reliably drive the relay when the 12 volts is being PWM'd, right?

Well, that has its own answer: Don't. That is, only actually drive the relay when PWM is not present. You need to be aware that relay contacts come in two flavors, normally open or normally closed, where "normally" means with the relay not activated.

So, use your filter/comparator to detect the presence/absence of PWM, and only drive the relay when the input is DC. Select your contacts to produce the result you want.

\$\endgroup\$
  • \$\begingroup\$ Correct, that solution should work without issue. My concern is being able to differentiate between the 12V constant state and the PWM state regardless of relay being utilized. I need to create a circuit that can discern between those two states and create a large enough voltage and current signal to drive a relay coil. Those are my concerns and ultimately, have the specifics to put together such a circuit. \$\endgroup\$ – C Hamilton Oct 26 '18 at 14:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.